Pneumatic rubber tires intended for heavy duty service conventionally have a tread with a cap/base layered construction where the outer rubber cap layer contains the running surface of the tread (intended to be ground-contacting) and where the composition of the outer tread cap rubber layer elastomer is comprised primarily of natural cis 1,4-polyisoprene rubber and is therefore natural rubber-rich. Such tire treads often contain a minor amount of cis 1,4-polybutadiene rubber to promote resistance to treadwear for the tire. Filler reinforcement for the tread rubber may be, for example, rubber reinforcing carbon black and may also include precipitated silica together with a coupling agent for the precipitated silica.
Polybutadiene rubber is often not considered as being suitable as a primary elastomer for heavy duty tire treads (e.g. truck tire treads) because of, for example, difficulty in processing uncured cis 1,4-polybutadiene rubber, particularly high viscosity cis 1,4-polybutadiene rubber.
Therefore, a challenge is presented for evaluating whether a synthetic rubber, namely synthetic cis 1,4-polybutadiene rubber, may be used to replace a significant amount of natural rubber normally used for such heavy duty tire treads, intended to be road contacting, to enhance processing of the uncured rubber composition in combination with achieving suitable resistance to tire treadwear (abrasion resistance) for the cured rubber composition, which are desirable rubber composition properties for the heavy duty tread cap rubber intended to be ground contacting, namely the outer, ground-contacting portion, or layer, of the tire tread.
For such evaluation, it is desired to evaluate utilization of a rubber composition containing synthetic functionalized cis 1,4-polybutadiene rubber containing filler reinforcement comprised of hydrophobated precipitated silica, particularly with a relatively minimal content of rubber reinforcing carbon black.
For such evaluation, a combination of functionalized cis 1,4-polybutadiene rubber containing functional groups reactive with hydroxyl groups on precipitated silica of the rubber reinforcing filler in combination with cis 1,4-polyisoprene rubber is to be undertaken.
Tire components such as tire sidewall and tread sub-tread (tread base rubber layer) have been suggested which contain precipitated silica and functionalized cis 1,4-polybutadiene rubber. For example, see U.S. Pat. No. 8,426,512.
However, this invention relates to a tire having a tread intended for heavy duty service in a sense of requiring its outer tread cap (the portion, or outer layer, of the tread intended to be ground-contacting) rubber composition to be comprised of functionalized cis 1,4-polybutadiene rubber and cis 1,4-polyisoprene rubber, particularly natural cis 1,4-polyisoprene rubber, and particularly where the rubber composition contains a significant content of the functionalized cis 1,4-polybutadiene rubber.
In one aspect, it is desired that the functionalized cis 1,4-polybutadiene rubber is exclusive of a tin or silicon coupled, cis 1,4-polybutadiene rubber.
The hydrophobated precipitated silica for this invention is contemplated as a precipitated silica pre-hydrophobated (prior to its addition to the rubber composition) or hydrophobated in situ within the rubber composition with a silica coupler comprised of a bis(3-trialkoxysilylalkyl) polysulfide with an average of from 2 to 3.8, alternately from 2 to about 2.6 or from about 3.4 to about 3.8, connecting sulfur atoms in its polysulfidic bridge, or is comprised of an alkoxyorganomercaptosilane. The bis(3-trialkoxysilylalkl) polysulfide may be, for example, comprised of bis(3-triethoxysilylpropyl) polysulfide.
In the description of this invention, terms such as “compounded rubber”, “rubber compound” and “compound”, if used herein, refer to rubber compositions containing of at least one elastomer blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms “elastomer” and “rubber” may be used herein interchangeably unless otherwise indicated. It is believed that such terms are well known to those having skill in such art.